docs: def nexus(network, before_state, after_state,
            direction=Direction.BIDIRECTIONAL):
      """Return a tuple of all irreducible nexus of the network."""
      validate.is_network(network)
  
      sias = (sia(transition, direction) for transition in
              transitions(network, before_state, after_state))
      return tuple(sorted(filter(None, sias), reverse=True))